Antenna arrangement and a method in connection with the antenna arrangement

ABSTRACT

An antenna device for emitting and/or receiving electromagnetic radiation, such as radar radiation in a car radar apparatus is provided. Sweeping is realized by means of a fixed feeder and a rotatable reflector. The reflector is driven by a drive motor and its motion is monitored by a tachometer. The drive motor and the tachometer are constituted by so-called flag motors which are mutually motionally coupled to each other by mechanical means.

The present invention relates to a method of, in an antenna arrangementwhich comprises a feeder network, a radiating element and a reflectorsystem and scans space in a first plane, bringing about increasedresolution in a second plane at right angles to the first plane. Theinvention also relates to an antenna arrangement forming part of anauxiliary system, working according to radar principles, for vehicles,which arrangement is connected to a signal source and/or a signalreceiver and comprises a feeder network, a radiating element and also areflector system for distributing output power from the radiatingelement or, respectively, focusing incoming radiation from space ontothe radiating element.

An antenna arrangement according to the above is previously known fromour earlier Swedish Patent 505 599. The antenna has a disc-shaped lobeand is intended to scan an area in the horizontal plane of the order of10-15 degrees. This is brought about by rotating the main reflector in areflector system of the Cassegrain type.

The known antenna arrangement has no resolution in the elevationdirection. It has, however, become desirable to be able to equip theknown type of antenna arrangement with some form of elevationresolution. By introducing elevation resolution, the antenna arrangementcan be used in order to distinguish stationary objects on the roadway,e.g. cars, from objects above the roadway, e.g. bridges and road signs.

The object of the present invention is to produce a method and anantenna arrangement which not only afford resolution in a first plane,preferably the horizontal plane, but also afford at least limitedresolution in a plane at right angles to the first plane, preferably thevertical plane. The object of the invention is achieved by a methodcharacterized in that the phase center of the radiating element is movedrelative to the reflector system in the elevation plane by dividing theradiating element and feeding the divided radiating element according toat least a first and a second power distribution model, and also anantenna arrangement characterized in that the radiating element isdesigned so as to be divided into at least two radiating part-elementsand in that the feeder network comprises a distribution network adaptedto distribute the signal power between the radiating part-elementsaccording to at least two different power distribution models.

In this context, phase center means a point in space, which is fixed inrelation to the antenna. For this point, it is ideally true that thewave radiated from the antenna has the same phase position on allspheres with their center point at this point. The point can also beregarded as the point where radiation takes place, i.e. the origin ofthe radiation. For the great majority of antennas, there is no suchpoint but, if the phase position is constant over that part of thesphere which is delimited by the main lobe, the center point of thesphere is still called the phase center of the antenna. For a moredetailed description of the phase center, refer to IEEE Standard TestProcedures for Antennas, ANSI/IEEE Std 149-1979, ISBN 0-471-08032-2.

The two power distribution models result in the generation of twodifferent lobes in elevation. In this connection, it is possible to makeone lobe, the normal lobe, point absolutely horizontally, while thesecond lobe, the elevation lobe, can point slightly upwards, e.g. 2-4degrees above the horizontal plane. Both the lobes can be scanned inazimuth.

The distribution of power between the divided radiating elements can beeffected stepwise or continuously according to the desired type of sweepof the lobe(s) of the antenna arrangement. According to a preferredembodiment, the power is distributed between the radiating part-elementsso that, for generating the elevation lobe, all the power is assigned toonly one radiating part-element while, for the normal lobe, the power isdistributed between two radiating part-elements so that bothpart-elements receive power.

The embodiment of the radiating element of the antenna arrangement mayvary in many respects. For example, according to an envisagedembodiment, the antenna arrangement may comprise a separate radiatingelement for receiving and a separate one for transmitting and either oneradiating element or both can be divided into two radiatingpart-elements. The possibility of varying the design of the radiatingelements within wide limits allows, inter alia, the cost of the totalnumber of components that are required to be kept down.

According to an advantageous embodiment, the radiating part-elementsconsist of horns. The reflector system of the antenna arrangement can beof the Cassegrain type for distributing or, respectively, focusing theradiation, and a combination of horns as radiating part-elements with areflector system of the Cassegrain type has proved to be advantageous.

In order to distribute the signal power between the radiatingpart-elements according to two power distribution models, thedistribution network comprises, according to another advantageousembodiment, a first and a second hybrid and a phase rotator, one outputon the first hybrid being connected directly to one input on the secondhybrid and the other output on the first hybrid being connected to theother input on the second hybrid via the phase rotator. The phaserotator can be variable. By varying the phase rotation in the variablephase rotator, the power can be distributed arbitrarily between twoassociated radiating part-elements. Advantageously, the phase rotatorcan be assigned a first and a second fixed position so that, in thefirst position, the power is distributed between the radiatingpart-elements and, in the second position, all the power is fed to oneof the radiating elements.

Advantageously, the phase rotator consists of a third hybrid providedwith a first and a second waveguide on the output side and a shortcircuit which can be introduced into each waveguide. So as to allowintroduction of the short circuits into the waveguides, the latter arepreferably provided with slots in two opposite delimiting surfaces atthose ends of the waveguides which are distant from the hybrid.According to the proposed embodiment, the short circuits can beintroduced by a linear movement or by a movement following the arc of acircle. According to a specific preferred embodiment, the short circuitsare arranged in association with the periphery of a rotatable circulardisc, the peripheral part of which passes through the slots of thewaveguides during rotation. In this connection, the circular disc canconstitute a part of the rotor in a motor included for rotating thedisc. The material in the disc should have a low dielectric constant.Expediently, the short circuits consist of metal strips, such as copperstrips.

By virtue of the embodiment according to the previous paragraph, theshort circuits can be made very much alike in the two waveguides andgood matching is achieved automatically. Integration of motor and dischas proved to be advantageous, inter alia with regard to moment ofinertia, power requirement and heat generation.

The hybrids can consist of 90 degree hybrids, but it is also possible tointroduce another hybrid type, for example 180 degree hybrids.

The invention is described in greater detail below by means of anexemplary embodiment with reference to the appended drawings, in which:

FIG. 1 shows a known reflector antenna of the Cassegrain type,

FIG. 2 shows diagrammatically in a front view and a side view aradiating element according to the principles of the invention, dividedinto two radiating part-elements,

FIG. 3 shows a diagrammatic sketch of the feeder network,

FIGS. 4a and 4 b show diagrammatically in a front view second and athird exemplary embodiment of the design of the radiating elements,

FIG. 5 shows a diagrammatic representation of a phase rotator couplingwith displaceable short circuits,

FIG. 6a shows an exemplary embodiment of a phase rotator coupling withlinearly displaceable short circuits,

FIG. 6b shows an exemplary embodiment of a phase rotator coupling withshort circuits which can be displaced along a circular arc,

FIG. 6c shows in a perspective view part of a waveguide included in aphase rotator coupling according to FIG. 6a or 6 b, and

FIG. 7 shows diagrammatically a phase rotator coupling according to FIG.6b integrated in a feeder network with horn antennas.

The known reflector antenna of the Cassegrain type shown in FIG. 1comprises a radiating element in the form of a horn 1, a main reflector2, and a subreflector 3, which two reflectors constitute the reflectorsystem of the reflector antenna. The subreflector 3, together with abottom part 4, a top part 5 and side walls (not shown in greater detail)form the outer delimitation of the antenna unit. The main reflector 2acts electrically like a plane here and the subreflector 3 is ofcylindrical parabolic shape. It may be pointed out that it is alsopossible to use a main reflector of appropriate curved shape, forexample parabolic shape. The antenna is horizontally polarized. Byinteraction between the horn and the reflectors, a disc-shaped lobe (notshown) is produced, which is intended to scan an area in the horizontalplane. The scanning in the horizontal plane is brought about by the mainreflector being arranged rotatably, for example ±6-7° about a verticalcentral axis, the antenna being steerable through ± double the angle inthe horizontal plane. A suitable frequency range for the antenna is76-77 GHz.

Furthermore, the main reflector rotates polarization by 90°. It is alsofocusing in the vertical direction. The subreflector has a focusingeffect in the horizontal direction. It is what is known as atransreflector, i.e. it acts in a reflecting manner with regard to onepolarization (linear vertical) while it is transparent with regard tothe orthogonal one (linear horizontal). Reflection of the verticalpolarization is brought about by means of a vertically etched strippattern. In the figure, the ray path has been shown by means of brokenlines 6, 7 and 8 and the E and H fields have been marked by arrows. Inthis connection, it can be seen from these indications that theradiation undergoes polarization rotation of 90 degrees in the mainreflector between the broken lines 7 and 8.

The reflector system 2, 3 is offset-fed via the radiating element in theform of a waveguide-based vertically polarized sectoral E-plane horn.The purpose of offset feeding is on the one hand to ensure a lowstanding-wave ratio, SWR, and on the other hand to avoid feeder blockingfor as long as possible.

If the antenna is regarded as a transmitting antenna, the operation ofthe antenna can be explained as follows: a vertically polarized waveinitiated from a signal source and originating from the feeder horn 1 isreflected in the subreflector 3 and is focused with regard to thehorizontal plane. The plane vertically polarized wave originating fromthe subreflector is reflected and is then rotated in polarization in themain reflector 2, after which it passes out through the subreflector.

A prerequisite for the lobe of the antenna described above to lie in thehorizontal plane is that the phase center of the feeder horn 1 lies atthe focal point of the reflector system. If the feeder horn 1 is movedslightly downwards so that the phase center of the horn is then belowthe focal point, the lobe will point slightly upwards. The distance fromthe focal point determines the angle of the lobe in the elevationdirection.

Our idea for achieving elevation resolution is based on dividing theradiating element into radiating part-elements. FIG. 2 showsdiagrammatically the division of a radiating element in the form of afeeder horn into two smaller horns 1.1, 1.2 separated in the verticaldirection, the left part showing a front view of the horns and the rightpart a side view of the horns. By distributing the power arbitrarilybetween the two horns, scanning in the elevation direction between amaximum and a minimum angle is brought about. In order to obtain a lobein the horizontal plane, the power is distributed between the two hornsso that the combined radiation from the horns has its phase center atthe focal point. If all the power is distributed to the lower horn, thelobe will point slightly upwards. According to a proposal for dividingthe feeder horn, the upper horn is provided with an aperture which is atleast twice the size of that of the lower horn by means of increasedextent in the vertical plane. Distribution of the available power sothat roughly −1.5 dB is fed to the upper horn and roughly −5.2 dB is fedto the lower horn can cause the phase center of the combined radiationto lie at the focal point. If all the power is fed to the lower horn,the phase center of the radiation will then be below the focal point.This leads to an elevation lobe being generated, which points a fewdegrees above the horizontal plane.

FIG. 3 shows a diagrammatic sketch of a suitable feeder network 9 forfeeding a divided radiating element in the form of a lower and an upperhorn 1.1, 1.2. The feeder network 9 is connected to a signal source 10in the form of a signal generator. The feeder network comprises adistribution network, which in the diagrammatic sketch is identical withthe feeder network, for distributing the power between the upper andlower horn. The distribution network comprises two 90 degree hybrids 11,12 and a variable phase rotator 13.

In this connection, hybrid means a component which divides incomingline-conducted microwave energy between two outgoing lines. The powervalues in the two outgoing lines are the same but are phase-rotated inrelation to one another. The hybrids are divided into two differentgroups depending on the phase difference in the two outgoing lines, thatis to say 90 degree hybrids and 180 degree hybrids, resulting in a 90degree or, respectively, a 180 degree phase difference between theoutgoing lines of the hybrid. The hybrids are in most cases providedwith two inputs and two outputs and are standard components for themicrowave designer.

Incoming power from the signal source 10 is fed into the first 90 degreehybrid 11, one output of which is connected directly to one input on thesecond 90 degree hybrid and the other output of which is connected, viathe phase rotator 13, to the other input on the second hybrid 12. Thefirst output of the second hybrid is connected to the upper horn 1.2 andthe second output is connected to the lower horn 1.1.

By varying the phase rotation in the variable phase rotator 13, theoutput power from the signal source 10 can be distributed arbitrarilybetween the two horns 1.1, 1.2. The variable phase rotator can be giventwo fixed positions, a first in which the power is distributed betweenthe upper and lower horn and a second in which all the power isdistributed to the lower horn. Switching between these two fixedpositions means that the antenna lobe is moved between two differentelevation angles, e.g. between a normal lobe in the horizontal plane,the first position, and an elevation lobe pointing a few degrees upwardsrelative to the horizontal plane, the second position.

In the event that the antenna functions as a receiving antenna, signalsreceived by the horns 1.1, 1.2 from free space are supplied via thedistribution network to a signal receiver (not shown) connected on thesame side of the distribution network as the signal source.

According to the front view of the radiating elements of the antennaarrangement in a second exemplary embodiment shown in FIG. 4a, theantenna arrangement has been provided with two radiating elements 14 and15, one being used for transmitting and the other for receiving. The tworadiating elements 14, 15 are divided into radiating part-elements 14.1,14.2 and, respectively, 15.1, 15.2 arranged one above the other. Thedistribution of power between an upper and a lower part-element thatbelong together takes place according to the same principles as weredescribed above for an embodiment with a common radiating element fortransmitting and receiving.

FIG. 4b shows a third exemplary embodiment of the design of theradiating elements. Like the exemplary embodiment according to FIG. 4a,the antenna arrangement has been provided with two radiating elements14, 15. One radiating element is intended for transmitting and the otherfor receiving. In this case, only one 14 of the radiating elements isdivided into part-elements 14.1, 14.2. Here, the radiating element 14can be used for transmitting and the radiating element 15 for receiving.The reverse, using the radiating element 15 for transmitting and theradiating element 14 for receiving, is also possible. By forgoingdividing one radiating element 15 into part-elements, the number ofcomponents required can be reduced. However, a reduced sweep in theelevation direction can be expected.

Examples of how the phase rotator 13 included according to FIG. 3 can bedesigned are described in greater detail below with reference to FIGS.5, 6 a, 6 b, 6 c and 7.

According to FIG. 5, which illustrates the principle itself, the phaserotation is effected by means of a hybrid 16 with a first and a secondwaveguide 17, 18 on the output side of the hybrid, into which waveguidesshort circuits 19, 20 can be introduced. In order to bring about shortcircuiting, it is only necessary to introduce a sufficiently large metalstrip into the middle of the E plane of the waveguide. On the inputside, the hybrid is connected to the first and second hybrid 11 and 12respectively in the manner shown in FIG. 3. By introducing the shortcircuits, the waveguides are shortened in terms of conduction and aphase shift is thus brought about in comparison with the situation whenthe waveguides are allowed to operate unshortened.

FIG. 6a shows an example of how the short circuits can be introducedinto the waveguides by means of a linear movement. A linearlydisplaceable plate 21 with a low dielectric constant is provided withshort circuits 19, 20 in the form of metal strips. The plate 21 isdisplaceable across two parallel waveguides 17, 18 in the directionindicated by the arrow 26. Parts of the plate 21 run in slots 22arranged in those ends of the waveguides which are distant from thehybrid. The design of the slots 22 can be seen best in the perspectiveview shown in FIG. 6c of one end of a waveguide. A first fixed positionof the phase rotator is defined by the short circuits being locatedcompletely outside the waveguides and a second position is defined bythe state shown in FIG. 6a where the short circuits are introduced intothe waveguides.

According to the exemplary embodiment shown in FIG. 6b, the waveguides17 and 18 are directed in towards the center of rotation 24 of arotatable disc 23 with a low dielectric constant. An arrow 27 indicatesthe rotary movement of the disc. Two short circuits 19, 20 are arrangedon the disc 23. Similarly to what has been described previously, thewaveguides are provided with slots 22. By rotating the disc about thecenter of rotation of the disc, the phase rotator can be made to adopttwo fixed positions, one in which the short circuits 19, 20 are locatedcompletely outside the waveguides and one in which the short circuitsare introduced into the waveguides. The rotation of the disc 23 can bebrought about by, for example, a stepping motor connected to the disc.Alternatively, the disc can be integrated into the rotor part of thestepping motor.

By way of suggestion, a suitable material with a low dielectric constantfor the plate 21 or, respectively, the disc 23 is Duroid 5880. When themetal strip is moved out of the waveguide, only material with a lowdielectric constant will be situated in the waveguide within the slot22, which does not have any appreciable effect on the waveguide. Thissituation means that the waveguide is not appreciably affected by theplate or the disc.

In FIG. 7, the phase rotator coupling according to FIG. 6b has beenintegrated into a feeder network 9 with horn antennas 1.1, 1.2. The disc23 with the metal strips 19, 20 is shown here in a position in which themetal strips lie completely outside the waveguides. The figure proposesa possible waveguide pattern. It may be noted in particular that afourth hybrid 25 has been added in order to bring about better matching.Both the third and fourth hybrids preferably consist of hybrids with 90degree phase rotation.

Using short circuiting according to the above, it is possible to bringabout very coordinated and uniform phase rotation with good matchingwithout any special measures.

The exemplary embodiment described above applying our inventive idea isnot to be regarded as limiting for the invention, but a number ofalternative embodiments are contained within the scope of the invention,as defined in the patent claims appended to the description. Thereflector system does not have to consist of a Cassegrain configuration,but other reflector systems are possible, such as, for example,different systems of single-curved, double-curved and/or planereflecting surfaces intended to distribute the power from the radiatingelement in a desired manner in space or alternatively to focus incomingradiation from space onto the radiating element. The radiating elementdoes not have to consist of horns, but all other types of radiatingelements can be considered, for example radiating elements based onpatch technology.

What is claimed is:
 1. Method of, in an antenna arrangement whichcomprises a feeder network, a radiating element and a reflector systemand scans space in a first plane, bringing about increased resolution ina second plane at right angles to the first plane, characterized in thatthe phase center of the radiating element is moved relative to thereflector system in the elevation plane by dividing the radiatingelement and feeding the divided radiating element according to at leasta first and a second power distribution model.
 2. Method according toclaim 1, characterized in that the distribution of power between thedivided radiating elements is switched in steps for stepwise movement ofthe lobe of the antenna arrangement.
 3. Method according to claim 1,characterized in that, according to the first power distribution model,all the power is distributed to only one divided part of the radiatingelement and in that, according to the second power distribution model,the power is distributed between the divided parts of the radiatingelement so that each divided part is assigned power according to a fixedmutual ratio.
 4. Method according to claim 1, characterized in that thedistribution of power between the divided radiating elements is changedcontinuously for generation of a continuous sweep of the lobe of theantenna arrangement.
 5. Antenna arrangement forming part of an auxiliarysystem, working according to radar principles, for vehicles, whicharrangement is connected to a signal source and/or a signal receiver andcomprises a feeder network, a radiating element and also a reflectorsystem for distributing output power from the radiating element or,respectively, focusing incoming radiation from space onto the radiatingelement, characterized in that the radiating element is designed so asto be divided into at least two radiating part-elements and in that thefeeder network comprises a distribution network adapted to distributethe signal power between the radiating part-elements according to atleast two different power distribution models.
 6. Antenna arrangementaccording to claim 5, characterized in that the radiating element isdivided into two part-elements arranged one above the other.
 7. Antennaarrangement according to claim 5, characterized in that the antennaarrangement comprises a separate radiating element for receiving and onefor transmitting and in that at least one of the two separate radiatingelements is designed so as to be divided into two radiatingpart-elements.
 8. Antenna arrangement according to claim 5,characterized in that the radiating part-elements consist of horns. 9.Antenna arrangement according to claim 5, characterized in that thedistribution network comprises a first and a second hybrid and a phaserotator, one output on the first hybrid being connected directly to oneinput on the second hybrid and the other output on the first hybridbeing connected to the other input on the second hybrid via the phaserotator.
 10. Antenna arrangement according to claim 9, characterized inthat the hybrids are 90 degree hybrids.
 11. Antenna arrangementaccording to claim 9, characterized in that the phase rotator isvariable.
 12. Antenna arrangement according to claim 9, characterized inthat the phase rotator comprises a first and a second fixed position, inwhich first position the power is distributed between the radiatingpart-elements and in which second position all the power is fed to oneof the radiating part-elements.
 13. Antenna arrangement according toclaim 9, characterized in that the phase rotator consists of a thirdhybrid provided with a first and a second waveguide on the output sideand a short circuit which can be introduced into each waveguide. 14.Antenna arrangement according to claim 13, characterized in that thoseends of the first and second waveguide which are distant from the hybridare provided with slots in two opposite delimiting surfaces of thewaveguides, which slots are dimensioned to allow introduction of shortcircuits.
 15. Antenna arrangement according to claim 13, characterizedin that the short circuits are arranged so as to be introduced into thewaveguides by a linear movement.
 16. Antenna arrangement according toclaim 13, characterized in that the short circuits are arranged so as tobe introduced by a movement following the arc of a circle.
 17. Antennaarrangement according to claim 16, characterized in that the shortcircuits are arranged in association with the periphery of a circulardisc which is rotatable about a center of rotation and the peripheralpart of which passes through the slots of the waveguides duringrotation.
 18. Antenna arrangement according to claim 17, characterizedin that the circular disc constitutes a part of the rotor in a motorincluded for rotating the disc.
 19. Antenna arrangement according toclaim 17, characterized in that the circular disc consists of a materialwith a low dielectric constant.
 20. Antenna arrangement according toclaim 13, characterized in that the short circuits consist of metalstrips, such as copper strips.
 21. Antenna arrangement according toclaim 5, characterized in that the reflector system is of the Cassegraintype for distributing or, respectively, focusing the radiation.